diff --git a/.gitmodules b/.gitmodules
index 8b176a4..af42d1b 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -7,3 +7,7 @@
 #
 # Copyright 2024 林博仁(Buo-ren Lin) <buo.ren.lin@gmail.com>
 # SPDX-License-Identifier: CC-BY-SA-4.0
+[submodule "九日 Nine Sols"]
+	path = 九日 Nine Sols
+	url = https://gitlab.com/libre-knowledge/nine-sols.git
+	ignore = dirty
diff --git a/README.md b/README.md
index f327bd9..95ee5eb 100644
--- a/README.md
+++ b/README.md
@@ -5,6 +5,13 @@
 <https://gitlab.com/libre-knowledge/computer-games>  
 [![GitLab CI 持續整合流程狀態標章](https://gitlab.com/libre-knowledge/computer-games/badges/main/pipeline.svg?ignore_skipped=true "點擊查看 GitLab CI 持續整合流程的運行狀態")](https://gitlab.com/libre-knowledge/computer-games/-/commits/main) [![「檢查專案中的潛在問題」GitHub Actions 作業流程狀態標章](https://github.com/libre-knowledge/computer-games/actions/workflows/check-potential-problems.yml/badge.svg "本專案使用 GitHub Actions 自動化檢查專案中的潛在問題")](https://github.com/libre-knowledge/computer-games/actions/workflows/check-potential-problems.yml) [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit&logoColor=white "本專案使用 pre-commit 檢查專案中的潛在問題")](https://github.com/pre-commit/pre-commit) [![REUSE 規範遵從狀態標章](https://api.reuse.software/badge/gitlab.com/libre-knowledge/computer-games "本專案遵從 REUSE 規範降低軟體授權合規成本")](https://api.reuse.software/info/gitlab.com/libre-knowledge/computer-games)
 
+## 子主題
+
+以下列舉本主題相關的主題：
+
+* [九日 Nine Sols](https://gitlab.com/libre-knowledge/nine-sols)  
+  由赤燭遊戲所推出之類魂、類銀河惡魔城遊戲
+
 <!--
 ## 基本概念
 
@@ -18,10 +25,6 @@
 
 （待補）
 
-## 子主題
-
-以下列舉本主題相關的主題：
-
 ## 參考資料
 
 以下列舉撰寫本主題內容時所參考的第三方資源：
diff --git "a/\344\271\235\346\227\245 Nine Sols" "b/\344\271\235\346\227\245 Nine Sols"
new file mode 160000
index 0000000..fcfece8
--- /dev/null
+++ "b/\344\271\235\346\227\245 Nine Sols"	
@@ -0,0 +1 @@
+Subproject commit fcfece8de4e6ff1e6a06844312d4d5d8221df3ac